1. Field of the Invention
The present invention relates to a dynamic type decoder circuit used for memory or register circuits, more particularly, to a decoder circuit which stabilizes the output level during the decoding period to a predetermined level (selected level or nonselected level) in accordance with address information.
2. Description of the Related Art
In general, usual dynamic type decoder circuits are provided with a plurality of decoding transistors to whose gates address signals are input, a first transistor to whose gate a clock signal is input and which is conductive at a reset period (for example, a precharging transistor), and a second transistor to whose gate another clock signal is input and which is conductive at a decoding period (active period) (for example, a discharge transistor). By this, each time the first transistor turns on during the reset period (at which time the second transistor turns off), the stray capacity at the node of the output side of the decoder circuit is precharged and the potential of the output side becomes high level.
Next, when a certain cycle period (1) is entered, in the decoding period of the first half of the cycle period (1), the decoding transistors directly connected to the output side node are turned off and the rest of the decoding transistors are turned on, in accordance with the address signals input to the decoding transistors. At this time, the potential of the output side node remains the above-mentioned high level (precharged level) and the decoder circuit is in the nonselected state, but the potentials of the connection nodes of the above-mentioned decoding transistors are made low level since all of the charges precharged to the stray capacities existing at the connection nodes are discharged through the above-mentioned second transistor, which is in the on state at that time. Note that the latter half of the cycle period (1) is the reset period (where the output side node is again precharged), but the potentials of the connection nodes remain at the above-mentioned low level.
Next, at the decoding period of the first half of the cycle period (2), the decoding transistors directly connected to the second transistor are turned off and the remaining decoding transistors are turned on, in accordance with the address signals input to the decoding transistors. At this time too, the potential of the output side node must be the above-mentioned high level (that is, the decoder circuit is in the nonselected state). However, at the prior cycle period (1), the potentials of the above-mentioned connection nodes all became low level and the stray capacities in the discharged state existing at the connection nodes become connected in parallel through the on state decoding transistors with the stray capacity of the precharged state existing at the output side node. In particular, when the ratio of the stray capacities existing at the connection nodes to the stray capacity existing at the output side node is high, a so-called capacitive division of voltage occurs during the decoding period and the potential of the output side node drops. Therefore, in the above-mentioned decoding period, despite the decoder circuit being in the nonselected state, there is a danger of erroneous recognition of the selected state where the potential of the output side node is the low level (the selected state occurs when all the decoding transistors turn on in accordance with the address signals input to the decoding transistors during the decoding period).
As mentioned above, in the above-mentioned usual dynamic type decoder circuit, the stray capacities existing at the connection nodes of the decoding transistors are connected through the on state decoding transistors with the stray capacity existing at the output side node during the decoding period (active period), whereby the above-mentioned capacitive division of voltage occurs and the potential of the output side node fluctuates. In particular, when the value of the stray capacities existing at the connection nodes is great, as mentioned above, the output side potential fluctuates strongly and may become a cause of misoperation (with the nonselected state being erroneously recognized as the selected state).